I. Field of the Invention
The present invention relates to spherical grains of rare earth oxides useful in the manufacture of sintered products of rare earth oxides, and also to a method of manufacturing the spherical grains of rare earth oxides.
II. Description of the Prior Art
Conventionally, spherical grains of rare earth oxides are manufactured by adding water and then an organic or inorganic binder to a powder of the rare earth oxides to form a slurry. The slurry is molded and simultaneously dehydrated by known methods, such as, the slip cast method or the rubber press method. The mold is then dried and baked. Methods of casting reactive metals into ceramic molds are disclosed by Feagin U.S. Pat. Nos. 4,740,246 and 4,787,439. However, the green density (pre-sintering density) of the material is much less than the after-sintering density of the resulting sintered body because the coefficient of contraction is high. This seriously deforms the mold as it is dehydrated. It is thus difficult to obtain high dimensional precision in the resulting sintered body. Furthermore, the deformed sintered body acquires so much residual stress that its impact strength is poor and it has a high tendency to crack and break. As a result, the yield of the sintered body is lowered.
In an attempt to solve these problems, a rare earth oxide powder was granulated to form spherical grains having a relatively large mean grain diameter. A fine powder was added to the granulate to fill in the voids among the spherical grains and thereby attain the closest packing of the green powder, which was then molded and sintered. However, the spherical grains obtained by spray-drying a commercially available rare earth oxide powder having a mean grain diameter of from 3 to 6 .mu.m are generally cavernous, and the thus obtained green powder has a relatively low density.